Effect of the stacking order, annealing temperature and atmosphere on crystal phase and optical properties of Cu2SnS3

Cu2SnS3 (CTS) is emerging as a promising absorber for the next generation thin film solar cells (TFSC) due to its excellent optical and electronic properties, earth-abundance and eco-friendly elemental composition. In addition, CTS can be used as precursor films for the Cu2ZnSnS4 (CZTS) synthesis. The optical properties of CTS are influenced by stoichiometry, crystalline structure, secondary phases and crystallite size. Routes for obtaining CTS films with optimized properties for TFSC are still being sought. Here, the CTS thin films synthesized by magnetron sputtering on soda lime glass (SLG) using Cu and SnS2 targets in two different stacks, were studied. The SLG\Cu\SnS2 and SLG\SnS2\Cu stacks were annealed in S and Sn + S atmospheres, at various temperatures. Both stacks show a polymorphic structure, and higher annealing temperatures favor the monoclinic CTS phase formation. Morphology is influenced by the stacking order since a SnS2 top layer generates several voids on the surface due to the evaporation of SnS, while a Cu top layer provides uniform and void-free surfaces. The films in the copper-capped stack annealed under Sn + S atmosphere have the best structural, morphological, compositional and optical properties, with tunable band gaps between 1.18 and 1.37 eV. Remarkably, secondary phases are present only in a very low percent (< 3.5%) in samples annealed at higher temperatures. This new synthesis strategy opens the way for obtaining CTS thin films for solar cell applications, that can be used also as intermediary stage for CZTS synthesis.


Results and discussion
shows the grazing incidence X-ray diffraction (GIXRD) results of the SLG\Cu\SnS 2 and SLG\SnS 2 \Cu stacks annealed at different temperatures (300, 350, 375, 400 and 500 °C) in sulfur atmosphere. The as-deposited samples are fully amorphous (data not shown) which means that the stacked films (Cu and SnS 2 ) are almost diffused 40 after deposition. The sulfurization at 300 °C produces a slow formation of crystalline Cu 2 SnS 3 nanoclusters (the diffraction peaks of the CTS phase are broad), as compared with the much faster reaction of sulfur atoms (from vapors or from SnS 2 ) with Cu atoms, which produces much larger clusters (sharper diffraction peaks) of the polycrystalline hexagonal CuS (h-CuS) phase 41 , space group P63/mmc (194) (ICDD 00-006-0464). In the SLG\SnS 2 \Cu stack, h-CuS is the dominant phase, while in the SLG\Cu\SnS 2 stack, h-CuS is a minor phase, which means that it is produced mainly by the sulfur atoms from atmosphere. In addition, few peaks of the CuSn 3.75 S 8 crystalline phase are also observed. This behavior is maintained by sulfurization at 350 °C, but the formation of CTS crystalline clusters is faster (the diffraction peaks of the CTS phase are much sharper) than the formation of h-CuS, and therefore the ratio of h-CuS phase is smaller in the SLG\SnS 2 \Cu stack, while in the SLG\Cu\SnS 2 stack, only traces of h-CuS are observed (identified by a small broad shoulder at 2θ = 27.1°). After annealing at 375 °C, h-CuS is barely visible in both stacks. At 400 °C, the same low quantity of h-CuS phase is maintained in the SLG\Cu\SnS 2 stack, while for the SLG\SnS 2 \Cu stack, h-CuS phase increases. After the final sulfurization at 500 °C, h-CuS increases again in both stacks, probably due to low Sn content in the films as a consequence of SnS evaporation 42,43 . However, this low quantity secondary h-CuS phase is usually on the surface of the films and can be removed by KCN etching 44 .
To analyze the polymorphism of CTS, in Fig. 1b are synthetically presented the angular positions of the most intense CTS peaks in relation to the ICDD cards of cubic CTS (CTS c ) (04-002-6009) 45 , monoclinic CTS (CTS m ) (04-010-5719) 46 , and tetragonal CTS (CTS t ) (04-009-7947) 47 structures. For the SLG\Cu\SnS 2 stack sulfurized at 300 °C, the positions of the most intense peaks are proportionally in relation with the monoclinic and tetragonal references. Between 350 and 500 °C, the main peaks are either a monoclinic structure or shifted between the CTS t and CTS m structures. For the SLG\SnS 2 \Cu stack, sulfurization at 300 °C produced a mixed CTS c -CTS m phase, at 350 °C the CTS t phase is formed, while between 375 and 500 °C the peaks are located between CTS t and CTS m structures. The monoclinic phase is considered to have lower defects and should be preferred as the highest solar cell PCEs are obtained in CTS absorber layers with this crystalline structure 48 .
Annealing using Sn + S vapors, was performed on both stacks at three different temperatures: 350, 500 and 550 °C. Figure 2 shows the GIXRD results. The lowest annealing temperature (350 °C) produces different results in the two stacks. Monoclinic and tetragonal CTS peaks are observed in SLG\Cu\SnS 2 Sn + S, while in SLG\SnS 2 \ Cu Sn + S a single CTS t structure is detected. At 500 °C and 550 °C, for both stacks, the CTS m phase is formed. However, it is difficult to firmly discern between the three structures due to their neighboring XRD patterns 16 34 . Thus, in the SLG\Cu\SnS 2 stack the intensity of the CTS peak at 2θ = 28.5° is decreasing with increasing temperature, while the h-CuS peak at 2θ = 27.1° became more prominent. On the other hand, in the SLG\SnS 2 \Cu stack the presence of the h-CuS phase is less evident. Therefore, a second structural technique, such as Raman spectroscopy is needed. The crystalline phases percentages were determined from GIXRD patterns. Crystallinity can be estimated by subtracting the sum of the crystalline peaks areas from the total area of the diffraction pattern 50 . Figure 3 exhibits the results obtained using this method, for the two stacks annealed in S and Sn + S atmospheres. The CTS phase percentage is as low as 61.23% and 29.91% in the SLG\Cu\SnS 2 S and SLG\SnS 2 \Cu S stacks, annealed at 300 °C. However, the CuS phase is more present in the sample with copper on top, which can be explained by the low annealing temperature, that causes the reaction of the Cu top layer with sulfur to form the CuS phase and the diffusion of a low amount of this phase to the SnS 2 bottom layer. There is a significant amorphous phase in the samples as can be observed in Fig. 3 from the difference between 100% and the percentage of crystalline phases. The sulfurization at 350 °C leads to an enhancement of the CTS crystallinity in both sample sets. The presence of the CuS phase is very low (2.06%) in the SnS 2 -capped sample, while in the Cu-capped film, it is 11.5%. By increasing the temperature to 400 °C the CuS phase decomposes, leading to an enhancement of CTS crystallinity percentage in the two sample stacks 51 . At 500 °C, the Cu 2 SnS 3 crystalline phase reaches the maximum value of 79.52% in the SLG\Cu\SnS 2 S sample and 87.71% in the SLG\SnS 2 \Cu S film, while CuS also increases. The two other series of samples annealed using Sn + S atmosphere show similar results. At 350 °C, the CTS phase crystallinity percentages are 69.11% and 67.88% in SLG\Cu\SnS 2 Sn + S and SLG\SnS 2 \Cu Sn + S, respectively, along with the presence of the CuS phase in very low percentages (5.47% and 2.53%, respectively). As the temperature increased to 500 and 550 °C, the percentage of the CTS crystalline phase improved 31 . The CuS phase percentages slightly increased and then decreased in the SLG\Cu\SnS 2 Sn + S stack, while in the SLG\SnS 2 \Cu Sn + S stack only small differences are noticed (between 2.5 and 3.5%). The CTS crystallinity in the SLG\Cu\SnS 2 Sn + S and The average crystallite size of the CTS films prepared using two different stacks annealed under S and Sn + S atmospheres was calculated using the Scherrer equation 52 , and represented in Fig. 4. It was found that the crystallite size is increasing with the temperature increase 31 , regardless of the used stack or annealing atmosphere. The crystallite size of both stacks SLG\Cu\SnS 2 S and SLG\SnS 2 \Cu S annealed using sulfur are between 3 and 15 nm for temperatures between 300 and 500 °C, respectively. Regarding the samples annealed under Sn + S atmosphere, the average crystallite size is increasing from 5 to 17 nm for the SLG\Cu\SnS 2 Sn + S stack, and from 12 to 18 nm for the SLG\SnS 2 \Cu Sn + S stack, when the temperature increased from 350 to 550 °C.
As discussed, the Cu 2 SnS 3 phase is known to be polymorphic and crystallize in monoclinic, cubic or tetragonal structures. It is very difficult to discern between the three CTS structures from GIXRD, especially between the CTS m and CTS c , when the peaks are broad due to nanometric size of the crystallites, as in our samples. Therefore, Raman spectroscopy was employed as a complementary investigation tool. However, there are still controversies in the assignment of the Raman modes of the three Cu 2 SnS 3 structures in the literature (see Table 1). Although several reports agree in the identification of the main Raman peak of each structure (i.e. 290 cm −1 for CTS m , 303 cm −1 for CTS c and 337 cm −1 for CTS t ) 26,53,54 , numerous contradictions have been reported in the assignment of other modes related to the three CTS structures. These confusions concern mainly the modes at 351 cm −1 that was attributed either to CTS c 27 or to CTS t 55-57 , and at 352 cm −1 assigned to CTS m 19,26,29,49,57,58 or to CTS t structure 59 . Also, the peak located at 354 cm −1 was identified belonging to monoclinic structure of the CTS phase 50 Fig. 5a,b, along with the Raman microscope images of the measured spots for each sample. The stacks prepared with Cu at the bottom have a similar mixed structure regardless of the annealing temperature (Fig. 5a), with several peaks characteristic to the monoclinic (257, 290, 317, 352 and 370 cm −1 ) 58 and the tetragonal (287,  Figure 5b shows the Raman spectra of the SLG\SnS 2 \Cu stack sulfurized at temperatures between 300 and 500 °C. Raman peaks of the CTS m were detected at 225, 257, 294, 352 and 369 cm −1 . The tetragonal structure, on the other hand, was identified by the peaks at 287 and 336 cm −1 . All samples contain both monoclinic and tetragonal structures, however, as it can be seen in Fig. 5b, the monoclinic structure is the main crystal system, except for the SLG\SnS 2 \Cu S 350 °C film. This sample is dominated by the Cu 2 SnS 3 tetragonal structure since three CTS t peaks (287, 297 and 336 cm −1 ) are observed along with a CTS m peak at 317 cm −1 . It is worth mentioning that small intensities at 303 and 356 cm −1 belonging to the CTS c were observed in almost all the samples, which might suggest the presence of this structure in very small amounts. One should note that a significant difference is observed in the Raman microscope images of the two stacks ( Fig. 5a,b). The SLG\SnS 2 \Cu samples appear to be denser and more compact compared to the SLG\Cu\SnS 2 samples, which can be explained by the evaporation of an important quantity of SnS from the samples with SnS 2 on top. The CuS phase was detected in the two stacks in all the samples by two peaks (264 and 474 cm −1 ) 67 when Raman spectroscopy was performed on large crystallites found on the surface of the samples annealed at low (300 °C) and high temperatures (500 °C) as shown in Fig. 6 using a 325 nm wavelength. The CuS is known to segregate on the surface in the form of easily visible large grains 68 . This confirms the conclusions of GIXRD analysis.  www.nature.com/scientificreports/ The Raman spectra of SLG\Cu\SnS 2 Sn + S and SLG\SnS 2 \Cu Sn + S samples annealed at 300, 500 and 550 °C under Sn + S atmosphere are presented in Fig. 7a,b, along with the optical images of the measured spots. It can be seen in Fig. 7a that the SLG\Cu\SnS 2 Sn + S stack annealed at 350 and 500 °C is composed mostly of the monoclinic Cu 2 SnS 3 phase, with the Raman peaks at 225, 257, 294, 317, 353 and 370 cm −1 . On the other hand, the SLG\Cu\SnS 2 Sn + S sample annealed at 550 °C contains the peaks of the CTS m phase along with an intense peak at 336 cm −1 and a smaller peak at 288 cm −1 which belong to the CTS t phase. The presence of the CTS t peaks can be related to the evaporation of SnS at high temperature leading to an increase in copper content. Similar conclusions were drawn in a study of the effect of Cu concentration on the structure of CTS thin films 55 , where the sample with the highest concentration of copper has several peaks belonging to the monoclinic CTS structure and a strong CTS t peak at 335 cm −1 . On the contrary, the stack with Cu on top show different results (Fig. 7b). The sample annealed at 350 °C consists of a dominant CTS t phase with peaks at 287, 298 and 335 cm −1 and a weak signal at 317 cm −1 of the CTS m phase. The two other samples annealed at higher temperature, 500 and 550 °C, are mostly composed of the CTS m identified by the peaks at 224, 257, 294, 316, 353 and 370 cm −1 . CTS c peaks at 303 and 356 cm −1 might exist in small quantities in all samples. One should note that the Raman spectra of all the samples annealed in Sn + S atmosphere were recorded in numerous spots, and no peak of the CuS phase was observed in any of the stacks except of a small shoulder at 264 cm −1 in the samples annealed at 500 and 550 °C. This can be explained by the presence of a small amount of this phase that couldn't be detected on the sample surface in the measured spots. All the identified or possible Raman peaks in both stacks annealed at different temperatures under S and Sn + S atmospheres are summarized in Table 2. Figure 8 shows the scanning electron microscopy (SEM) images of the SLG\Cu\SnS 2 S stack sulfurized at temperatures between 300 and 500 °C. It can be seen in Fig. 8a-e that the surface morphologies are inhomogeneous with voids (voids are visible only in Fig. 8b,c) in almost all the samples, due to the evaporation of SnS. The SEM image of the film sulfurized at 300 °C, in Fig. 8a shows a surface with grains of different shapes and sizes. As the temperature increases, surface of the films is more homogeneous and the particles become smaller and more monodispersed in size (Fig. 8b-d). The morphology of the film annealed at 500 °C, (Fig. 8e) exhibits a compact surface with small grains, however, few bigger particles are also present. This inhomogeneity in particle sizes and shapes is correlated with the existence of the CuS phase 69 as indicated previously in the GIXRD and Raman analyses.
The surface morphologies of the SLG\SnS 2 \Cu S films annealed at different temperatures are represented in the Fig. 9. From the SEM images it can be seen that the films are more homogeneous with fewer cracks or voids (voids are visible only in Fig. 9b,c) are present when compared to the previous stack shown in Fig. 8. As observed by GIXRD and Raman, all the samples contain large grains belonging to the CuS phase. The dominance of this secondary phase is evident in the samples SLG\SnS 2 \Cu S annealed at 300 °C. On the contrary, in the other samples, there are smaller particles rather than large particles present on the surface. The morphology of all films is compact and the size of the grains decreases by increasing the temperature. Smooth, compact and void-free morphology produces efficient charge generation and separation and it is responsible for enhanced performance of TFSC 70 . Figure 10a-c illustrate the SEM results of the SLG\Cu\SnS 2 Sn + S stack annealed at 350, 500 and 550 °C in Sn + S atmosphere. All films show sparse and dispersed surfaces, however, no voids are observed in this series of samples unlike the same stack annealed using sulfur (shown in Fig. 8). The use of an Sn + S atmosphere reduces the voids since the evaporated SnS is compensated by the Sn + S vapors 71 . The film annealed at 350 °C exhibits a www.nature.com/scientificreports/ www.nature.com/scientificreports/ homogeneous morphology with monosized grains. As the temperature increases to 500 and 550 °C (Fig. 10b,c) nonhomogeneous surfaces with different particle sizes and shapes are seen. Figure 11a-c reveal the morphology of the CTS films from the SLG\SnS 2 \Cu Sn + S stack at different temperatures using Sn + S atmosphere in the heating treatment. All films show dense and homogeneous surfaces without voids or cracks. Also, CuS grains are not visible from the SEM images, which might be due to the small quantity of this secondary phase in the films 72 . In fact, in the GIXRD analysis the intensity of the peak corresponding to CuS in the SLG\SnS 2 \Cu Sn + S stack, is lower than in the other set of samples (SLG\Cu\SnS 2 ) with the same heat treatment. Moreover, Raman spectroscopy did not identify any CuS phase in this stack. Figure 12a,b present the energy dispersive X-ray spectroscopy (EDS) measurements for the as-deposited and sulfurized samples using the two stacking configurations. The results of the SLG\Cu\SnS 2 S stack, annealed between 300 and 500 °C, are displayed in Fig. 12a. The evolution of the elemental composition of the CTS samples is characterized by a small increase followed by a decrease in Sn content when increasing the temperature. On the other hand, the copper percentage decreases at low temperatures (300, 350, 375 °C), and rises at 400 and 500 °C. On the contrary, S content is growing by increasing the temperature and reaches 50% at 400 °C (the intended percentage in stoichiometric Cu 2 SnS 3 ), then slightly decreases. As observed previously, SnS is evaporated at higher temperatures in the SnS 2 -capped samples 73 , which explains the decrease in tin and sulfur content in the films annealed above 375 °C. Almost all films are Cu-rich, Sn-rich and S-poor, with the films sulfurized at 350 and 375 °C (Cu ≈ 31.32%, Sn ≈ 18.36%, and S ≈ 50.32%) being the closest to the intended stoichiometry (Cu: 33.33%, Sn: 16.67%, and S: 50%). In Fig. 12b are represented the atomic percentages of the SLG\SnS 2 \Cu S samples sulfurized between 300 and 500 °C. In this stack the elemental composition is changing arbitrarily with increasing temperature. However, almost all the samples are Cu-rich and S-poor except for the samples sulfurized at 350 and 375 °C, with the latter being closer to the CTS stoichiometry (2:1:3). The elemental composition of the SLG\SnS 2 \Cu S stack annealed at 375 °C is Cu 1.90 Sn 1.08 S 3.02 . The Cu rich phases lead to higher electrical conductivity, while the Cu poor phases affect the optical quality (bandgap) of CTS compounds. Therefore, precise elemental control is necessary to obtain a pure CTS compound 44 . Figure 13a,b show the composition of the as-deposited and annealed CTS samples in Sn + S atmosphere. The as-deposited SLG\Cu\SnS 2 in Fig. 13a is characterized by a Cu-rich, Sn-rich and S-poor composition. By increasing the temperature between 350 and 550 °C, the copper content decreased while tin and sulfur atomic percentages increased. Copper decreased from 44.1% to stoichiometric values (between 31.6 and 33.6%). On the other hand, a tiny growth (between 20.9 and 21.5%) of the tin content is noticed, which is above the expected values. The S-poor composition observed in the as-deposited sample turns into a S close to stoichiometry in the annealed CTS films, reaching up to 47%. The sample annealed at 550 °C (Cu 1.93 Sn 1.27 S 2.80 ) is the one approaching the Cu 2 SnS 3 stoichiometry. One should note that the SLG\Cu\SnS 2 Sn + S stack shows an enhancement in both Sn and S concentration, while annealing the same stack using only S leads to decreases of these elemental ratios. This demonstrates that using Sn + S during the heat treatment can prevent the deficiency in Sn and S 42 . The EDS results of the as-deposited and annealed SLG\SnS 2 \Cu Sn + S stack are shown in Fig. 13b. Before annealing the film is Cu-rich, Sn-rich and S-poor. At temperatures between 350 and 550 °C, the copper and tin content decreases, while sulfur is enhanced. However, it is worth mentioning that all the samples are close to the Cu 2 SnS 3 stoichiometry, especially the SLG\SnS 2 \Cu Sn + S annealed at 550 °C (Cu 2.01 Sn 1.05 S 2.94 ).
In Fig. 14a,b are shown the band gap energies (E g ) of the sulfurized samples, using the two stacking configurations. The SLG\Cu\SnS 2 S samples annealed between 300 and 500 °C, show a decrease in the E g values when increasing the temperature (Fig. 14a). The sample annealed at 300 °C has the highest E g (1.48 eV), due to the dominance of the CuS phase, which is known to have a E g between 1.8 and 2.2 eV 74 . Starting from 350 °C, the E g decreases to 1.36, 1.37, 1.27 and 1.14 eV for the 350 °C, 375 °C, 400 °C, and 500 °C annealing temperatures, respectively. This happens because of the tetragonal and monoclinic mixed nature of the samples. The reported E g of the monoclinic and cubic structures is around 0.95, and the tetragonal structure is characterized by a E g www.nature.com/scientificreports/ of 1.35 eV 53,54 . As seen in Fig. 14b, the sample sulfurized at 300 °C using the SLG\SnS 2 \Cu S stack has an E g of 1.45 eV. The same conclusions can be drawn here. For the CTS films sulfurized at 350 and 375 °C, the estimated E g is between 1.32 and 1.30 eV, respectively. These two samples are the closest to the CTS t structure as demonstrated by the structural results. The SLG\SnS 2 \Cu S sample annealed at 400 °C has the lowest E g from all the samples (0.98 eV), which means that this sample is mainly composed of a monoclinic structure. At 500 °C, the E g increases again to reach 1.19 eV, maybe due to few CuS particles found in this film. In Fig. 15a,b are illustrated the E g of the SLG\Cu\SnS 2 Sn + S and SLG\SnS 2 \Cu Sn + S stacks annealed under Sn + S atmosphere. The E g values, ranging from 1.18 to 1.37 eV for the samples annealed between 350 and 550 °C, are increasing with the annealing temperature in the SnS 2 -capped samples. This can be interpreted by the structural transformations of the CTS samples with the increase of temperature, since the SLG\Cu\SnS 2 Sn + S samples are mainly composed of a mixed monoclinic-tetragonal structure. On the contrary, the SLG\SnS 2 \Cu Sn + S stack shows a decrease in the band gap values by increasing the temperature. The tetragonal structure is mostly present in the sample annealed at 350 °C, with a computed E g of 1.30 eV, which is close to the reported value for this structure. The samples annealed at 500 and 550 °C are characterized by lower band gaps (1.06 and 1.07 eV, respectively), due to their polymorphic structure.  www.nature.com/scientificreports/

Conclusions
In summary, eco-friendly Cu 2 SnS 3 thin films were synthesized by annealing two stacks (SLG\Cu\SnS 2 and SLG\ SnS 2 \Cu) in two different atmospheres: S only at 300, 350, 375, 400 and 500 °C, and Sn + S at 350, 500 and 550 °C. For the sulfurized samples, Raman spectroscopy demonstrated that all samples in the SLG\Cu\SnS 2 S stack are polymorphic. On the other hand, the SLG\SnS 2 \Cu S stack is characterized by a tetragonal dominant structure in the film annealed at 350 °C, while the films sulfurized at higher temperature are polymorphic with more peaks belonging to CTS m . The GIXRD measurements revealed the formation of the h-CuS secondary phase in both stacks sulfurized at 300 °C, and starting from 350 °C, this phase is present only as a trace, regardless of the stacking order. The sulfurized SnS 2 -capped samples exhibit a poor surface morphology with multiple voids due to SnS evaporation from the top layer. This was confirmed by the gradual decrease in Sn and S content observed in the EDS analysis. On the contrary, the copper-capped films have uniform and homogeneous surfaces with nearly stoichiometric compositions. The optical measurements on the sulfurized samples showed a dependence   For the annealing under Sn + S atmosphere at different temperatures, the SLG\Cu\SnS 2 Sn + S samples with SnS 2 as capping layer, contain very small amount of the h-CuS phase, as inferred from GIXRD and Raman spectroscopy. SEM images show a non-uniform surface, and a small increase in the Sn and S content is observed due to the Sn + S annealing conditions. The optical energy gap is between 1.18 and 1.37 eV. On the other hand, the SLG\SnS 2 \Cu Sn + S stack annealed at different temperatures using Sn + S atmosphere, possesses overall the most adequate properties. GIXRD measurement of the film annealed at 350 °C showed a CTS phase with oriented peaks towards the tetragonal structure. This was supported by a strong peak at 335 cm −1 observed using Raman spectroscopy. The samples annealed at 500 and 550 °C are polymorphic with the main Raman peaks belonging to the CTS m structure. The surface morphology is dense and compact in all films and the elemental compositions are the closest to stoichiometry, with E g between 1.06 and 1.30 eV.
This new synthesis strategy has revealed that Cu 2 SnS 3 films can be obtained from two different stacking configurations using Cu and SnS 2 sputtering targets. The surface morphology can be enhanced using a Cu-capped layer to avoid SnS evaporation, the structural parameters can be controlled to obtain the desired crystalline phase by varying the temperature, while Sn + S annealing is found to be beneficial against SnS losses and helps obtain the intended stoichiometry. These findings are helpful for the synthesis of CZTS films, by the solid reaction of an additional ZnS layer on top of CTS films.

Materials and methods
The thin films were synthesized onto soda lime silica glass (SLG) substrates using two sets of magnetron sputtering depositions. Two stacking configurations were obtained. For the first stack, SLG\Cu\SnS 2 , the Cu layer was sputtered first, followed by the deposition of a SnS 2 film on top. For the second stack, SLG\SnS 2 \Cu, the SnS 2 layer was sputtered first, followed by the deposition of a Cu film on top. The films were prepared at room temperature by radio frequency (RF) magnetron sputtering (3G Circular Gencoa) using SnS 2 and Cu sputtering targets (Mateck Gmbh, 99.99% purity). The stacked films were deposited without breaking the vacuum. The base pressure inside the deposition chamber was 4 × 10 -6 Torr before deposition and 5 × 10 -3 Torr during deposition, in an Argon environment with a gas flow of 30 standard cubic centimeters per minute (sccm). The  www.nature.com/scientificreports/ substrates were continuously rotated during deposition and placed at a distance of 11 cm from the sputtering targets. Before depositions, the sputtering rates of the magnetrons were optimized using an Inficon Q-bridge deposition monitor connected to a quartz microcrystal. The sputtering power was set at 50 W for Cu, and at 25 W for SnS 2 , leading to a sputtering rate of 0.85 Å/s for Cu and 0.67 Å/s for SnS 2 , respectively, using two RF-T&C Power Conversion Inc., Model AG 0313 sources. Prior to the deposition process, a 10 min pre-sputtering was performed on each target to eliminate any undesired impurities from the target surfaces. During deposition, the Cu target was sputtered for 1176 s to obtain a 100 nm thickness Cu film and the SnS 2 target was sputtered for 3720 s to obtain a 250 nm thickness SnS 2 film. A schematic of the sputtering chamber and the deposition parameters are presented in Fig. 16 and Table 3. The as-deposited samples were then loaded into a quartz tube with sealing flanges inside a GSL 1600X tubular furnace, and annealed at different temperatures for 10 min with a heating rate of 10 °C/min and a cooling rate of 5 °C/min. Several results have reported that the sulfurization time plays a crucial role in controlling the elemental Sn loss from the absorber compound. Prolonged sulfurization leads to higher Sn loss 75 . The SLG\Cu\ SnS 2 and SLG\SnS 2 \Cu stacks were annealed in two different atmospheres, using as source: a sulfur powder (S) only, or a mix of tin plus sulfur powders (Sn + S). An Argon flow of 10 sccm was used to ensure the reactive gasses transport to the samples surface. Samples were annealed at temperatures between 300 and 550 °C, since above this temperature the formation of non-compact morphology attributed to the elemental Sn loss was reported 76 . The samples were named according to the stacking order, annealing atmosphere and temperature.
The structural investigations were performed by grazing incidence X-ray diffraction (GIXRD) at an incidence angle of 0.3°, using a Rigaku SmartLab diffractometer in a parallel beam configuration, provided with Cu Kα www.nature.com/scientificreports/ radiation (λ = 1.54178 Å) and HyPix-3000 2D Hybrid Pixel Array Detector (in 0 D mode). For Raman spectroscopy a LabRAM HR Evolution Raman spectrometer from HORIBA Jobin-Yvon, equipped with a confocal microscope and a He-Ne laser, using red and UV excitation wavelengths of 633 and 325 nm, was employed. The laser was focused using an Olympus 100× objective on the sample surfaces. The surface morphology and elemental composition of the films were measured using a Zeiss EVO 50 XVP scanning electron microscope (SEM) coupled with a Bruker Quantax 200 detector energy dispersive spectrometer (EDS). Optical transmission measurements were carried out by a V-Vase Woollam Spectroscopic Ellipsometer accessorized with a high-pressure Xenon discharge lamp integrated in a HS-190 monochromator. The transmittance was measured by conventional spectroscopy in specular mode, and the optical band gaps were calculated using the following equations 77 : where α is the absorption coefficient, hν is the photon energy, A is the constant of proportionality, d is the film thickness (which is approximately 500 nm for all films), E g is the optical band-gap, and T is the recorded transmittance. The E g was estimated using the Tauc plot by plotting (αdhν) 2 vs. (hν) near the absorption edge. Since the thickness of the films is around 500 nm, a value of 20 for the product (αdhν) 2 is corresponding to an absorption coefficient of 2.3 × 10 4 cm −1 . 10 4 is a reference α value close to the band gap. Thus, the fitting of the experimental points should be performed in the range 0 to 20 in (αdhν) 2 .

Data availability
The data presented in this study are available on a reasonable request from the corresponding author.